Such transconductance amplifier circuits are known, and have served the commercial purpose of providing a controlled current source, which may be required in a number of practical applications, one example being the testing and calibration of wattmeters.
The basic requirement of a transconductance amplifier in precision measurements is that it faithfully reproduce the input voltage waveform as a current. This result has traditionally been achieved by measuring the output current with a resistive shunt and using the shunt voltage drop as a feedback signal to compare with the input. However, when a large output current is involved, it is difficult to construct a shunt that will have sufficient magnitude stability and a sufficiently low phase angle error. The current comparator is an instrument that has been used as a means to overcome this problem, its very accurate ratio characteristics enabling a high value resistor with small magnitude and phase errors to be used as a reference.